Real-time crystallization in fluorinated parylene probed by conductivity spectra

摘要

Dielectric relaxation spectroscopy experiments were performed at high temperature on fluorinated parylene films during the occurrence of the isothermal crystalline phase transition. For this polymer, since the difference between the glass transition temperature (T_g) and the phase transition temperature (T_c) is very strong (T_c ≥ 4T_g), segmental and dipolar relaxation usually used to probe the crystallization are not shown in the experiment frequency window (10~(-1) to 10~6Hz) during the crystallization. The charge diffusion becomes the only electrical marker that allows probing the phase transition. During the transition phase, a continuous decrease of about two orders of magnitude is observed in the conductivity values below an offset frequency (f_c) with a tendency to stabilization after 600 min. Below the offset frequency, the decrease of the normalized conductivity to the initial value as function of time is frequency independent. The same behavior is also observed for the f_c values that decrease from 160 Hz to about 20 Hz. Above the offset frequency, the electronic hopping mechanism is also affected by the phase transition and the power law exponent (n) of the AC conductivity shows a variation from 0.7 to 0.95 during the first 600 min that tend to stabilize thereafter. Accordingly, three parameters (n, f_c, and AC conductivity values for frequencies below f_c) extracted from the AC conductivity spectra in different frequency windows seem suitable to probe the crystalline phase transition.